Fibers of certain thermoplastic materials are used widely in the manufacturing of thermally bonded products, such as nonwoven textiles, by various processes. Said processes, such as calendering and spun bonding, require that the fibers have the capability of thermally bonding at temperatures lower than the melting point of the particular polymer(s) from which they are made, and that the fibers and articles manufactured therefrom be resistant to aging, yellowing and color variations caused by gas fading and oxidation.
There have been various attempts made to improve the thermal bondability of fibers, such as incorporating additives into the fiber grade polymer, elevating of spinning temperatures, forming fibers having two components and modifying the fiber surface. For example, U.S. Pat. No. 4,473,677 to Pellegrini et al discloses adding a dianhydride of a 3,3', 4,4' benzophenone tetracarboxylic acid or an alkyl derivative thereof to polyolefins to improve the thermal bonding of the fibers prepared therefrom. However, substantial problems are encountered during spinning at elevated temperatures and relatively slow spinning speeds are required.
Another approach is to add to the fiber grade polymer a low melting material, such as oligomers and waxes. The disadvantage of this approach is that the process must be modified to ensure adequate mixing of the materials so that gels are not formed in the fiber.
In the approach where fibers are formed from two different polymers, one component of the fiber has a lower melting point than the other, and covers the surface of the other component which has a higher melting point. These fibers are generally referred to as a "sheath-core" or "side-by-side" bicomponent fibers. The lower melting component enables thermal bonding at a temperature below the melting point of the fiber core.
Another approach is to modify the surface of the fiber once the fiber has been formed. Typically, these fibers contain only one fiber grade polymer, such as "skin fiber". Modification of the fiber surface can be obtained using various methods, such as irradiation, plasma treatment, ozone treatment, corona discharge treatment or chemical treatment.
In the typical process of melt spinning, the polymer is heated in an extruder to the melting point and the molten polymer is pumped at a constant rate under pressure through a spinneret containing one or more orifices of desired diameter, thereby producing filaments of the molten polymer. The molten polymer filaments are fed downward from the face of the spinneret into a cooling stream of gas, generally air. The filaments of molten polymer are solidified as a result of cooling to form fibers. Depending upon the spinning method used, the fibers are spread to form a fiber web and bonded directly, like in the spun bond method. Alternatively, in long spin methods, the fibers are gathered together and, if desired, drawn to orient the macromolecular structure of the fibers, and are then wound on bobbins. Bonding or calendering is then performed in a separate step. Generally, if there is any type of modification to be done to the filaments or fibers, such as surface modification carried out by chemical treatment or radiation treatment, the modification of the filaments or fibers takes place after the molten polymer filaments have solidified as a result of cooling to form the fiber, or on the preformed fiber itself.
It has now been found that the thermal bondability of fibers can be enhanced by treating the fiber grade polymer during the formation of the filaments, instead of treating the filaments or fibers after they are formed. The process of the present invention is not limited to any specific fiber preparation technique where a resin is melted and formed into a fiber, such as long spin, short spin, spun bond and melt blown fiber production methods. Nor is the spinning process limited to being carried out in any particular spinning environment, e.g. the presence or absence of oxygen or nitrogen.